The present invention relates to compounds and methods for treating malignant tumors, in particular brain tumors and tumors of the head and neck, using such compounds.
Porphyrins in general belong to a class of colored, aromatic tetrapyrrole compounds, some of which are found naturally in plants and animals, e.g., chlorophyll and heme, respectively. Porphyrins are known to have a high affinity to neoplastic tissues of mammals, including man. Because of their affinity for neoplastic tissues, in general, porphyrins with boron-containing substituents could prove useful in the treatment of primary and metastatic tumors of the central nervous system by boron neutron capture therapy (BNCT). Porphyrins and other tetrapyrroles with relatively long singlet lifetimes have already been used to treat malignant tumors with photodynamic therapy (PDT), but this application has limited clinical applicability because of the poor penetration of the visible light required to activate the administered enhancer so as to render it toxic to living tissues, i.e., to the targeted tumor.
Porphyrins have the added potential advantage of being useful in vivo as chelating agents for certain paramagnetic metal ions to achieve higher contrast in magnetic resonance imaging (MRI). They could also be chelated with radioactive metal ions for tumor imaging in single-photon-emission computed tomography (SPECT) or position emission tomography (PET). In principle, porphyrins could also be used for high-specific-activity radioisotope therapy when the carrier molecule can be targeted with sufficient biospecificity to the intended lesion so as to avoid normal tissue radiotoxicity, which is most often encountered, when present at all, in the bladder, bone marrow, liver, and lung—the likely sites of undesired bioaccumulation of unbound carrier or its degradation products.
Boron neutron-capture therapy (BNCT) is a bimodal cancer treatment based on the selective accumulation of a 10B carrier in tumors and subsequent irradiation with thermalized neutrons. The production of microscopically localized high linear-energy-transfer (LET) radiation from capture of thermalized neutrons by 10B in the 10(n, α)7Li reaction is responsible for the high efficacy and sparing of normal tissues. More specifically, the stable nuclide 10B absorbs a thermalized neutron to create two mutually recoiling ionizing high-energy charged particles, 7Li and 4He, with microscopic ranges of 5 μm and 9 μm, respectively.
When BNCT is used to treat patients experimentally with malignant tumors, the patient is given a boron compound highly enriched (≈95 atom %) in boron-10. The boronated compound is chosen based on its ability to concentrate preferentially in the tumor within the radiation volume. In the case of brain tumors, after injection of the boron compound, the patient's head is irradiated in the general area of the brain tumor with an incident beam or field of epithermal (0.5 eV-10 keV) neutrons. These neutrons become progressively thermalized (average energy approximately 0.04 eV) as they penetrate deeper into the head. As the neutrons become thermalized, they can more readily be captured by the boron-10 concentrated in the tumor cells and/or tumor supporting tissues, since the capture cross section is inversely proportional to the neutron velocity. A minuscule proportion of the boron-10 nuclei in and around a tumor undergoes a nuclear reaction immediately after capturing a neutron, which is why such a large concentration of boron-10 is required in and/or around a targeted cell or tissue for BNCT to be clinically effective. The present invention, when implemented clinically alone or in combination with existing or other new therapies, will meet this ‘high-concentration without undue toxicity’ requirement better than previously known compounds. This nuclear reaction produces the high linear energy transfer (LET) alpha (4He) and lithium (7Li) particles. The tumor in which the boron-10 is concentrated is irradiated by these short range particles which, on average, travel a distance comparable to, or slightly less than, the diameter of a typical tumor cell. Therefore, a very localized, specific reaction takes place whereby the tumor receives a large radiation dose compared with that received by surrounding non-neoplastic tissues, with relatively low boron-10 concentrations.
For BNCT of malignant brain tumors, it is particularly important that there be robust uptake of boron in tumor relative to normal tissues (i.e., blood and normal brain tissues) within the neutron-irradiated target volume. BNCT was used clinically at the Brookhaven National Laboratory Medical Department with p-boronophenylalanine (BPA) as the boron carrier (Chanana et al., Neurosurgery, 44, 1182-1192, 1999). BPA has the outstanding quality of not eliciting any chemical toxicity associated with its usage. However, because the brain and blood boron concentrations are approximately one-third those found in tumor, the tumor dose is restricted. In order to improve upon the currently used boron delivery agent, BPA, it has been postulated that tumor boron concentrations should be greater than 30 μg B/g and tumor:blood and tumor:brain boron ratios should be greater than 5:1 (Fairchild and Bond, Int. J. Radiat. Oncol. Biol. Phys., 11, 831-840, 1985, Miura, et al., Int. J. Cancer, 68, 114-119, 1996).
In PDT of malignant tumors using porphyrins, the patient is injected with a photosensitizing drug. The drug localizes preferentially in the tumor within the irradiation volume. The patient's tissues in the zone of macroscopic tumor is then irradiated with a beam of red laser light. The vascular cells of the irradiated tumor and some of the tumor cells are rendered incapable of further mitotic activity or may be killed outright if the light penetrates the tissue sufficiently. The biochemical mechanism of cell damage in PDT is believed to be mediated largely by singlet oxygen. Singlet oxygen is produced by transfer of energy from the light-excited porphyrin molecule to an oxygen molecule. The resultant singlet oxygen is highly reactive chemically and is believed to react with and disable cell membranes. Macroscopically, there appear to be some direct damage to tumor cells, extensive damage to the endothelial cells of the tumor vasculature, and infiltration of the tumor by macrophages. The macrophages remove detritus of dead cells from the PDT-treated zones of tissue, and in the process, are believed to damage living cells also.
In PDT, the drugs must be selectively retained by tumors, especially within the irradiation volume. However, the drugs should be non-toxic or minimally toxic when administered in therapeutically useful doses. In addition, drugs with absorbance peaks at long wavelengths allow increased tissue penetration and, thereby, allow photoablation of some or all of the vasculature and/or the parenchyma of deeper-seated tumors.
While it is well known in medical arts that porphyrins have been used in cancer therapy, there are several criteria that must be met for a porphyrin-mediated human cancer radiation treatment to be optimized. In BNCT, the porphyrin drug should deliver a therapeutically effective concentration of boron to the tumor while being minimally toxic to normal vital tissues and organs at a radiotherapeutically effective pharmacological whole-body dose of porphyrin. In addition, the porphyrin should have selective affinity for the tumor with respect to its affinity to surrounding normal tissues within the irradiation volume, and should be capable of achieving tumor-to-normal-tissue boron concentration ratios greater than 5:1. In vivo studies have shown that the latter criterion can be satisfied for brain tumors if the porphyrin, properly designed, synthesized and purified, does not penetrate the blood-brain barrier in non-edematous zones of the normal CNS.
In addition, if the boron concentration and distribution in and around the tumor can be accurately and rapidly determined noninvasively, BNCT treatment planning can be more quickly, accurately, and safely accomplished. For example, neutron irradiation could be planned so that concurrent boron concentrations are at a maximum at the growing margin of the tumor rather than in the tumor as a whole. Thus, BNCT could be implemented by one relatively short exposure or by a series of short exposures of mainly epithermal neutrons, appropriately timed to take advantage of optimal boron concentrations identified by SPECT or MRI in tumor, surrounding tissues, and blood in vivo. BNCT effectiveness in vivo is probably not diminished even when a neutron exposure is as short as 300 milliseconds. Such short irradiations have been delivered effectively, in fact, by a TRIGA (General Atomics) reactor operating in the pulse mode. Mice bearing advanced malignant sarcomas transplanted subcutaneously in the thigh were palliated and in many cases cured by BNCT using 300 millisecond ‘pulse’ exposures to slow neutrons (Farr, L. E., BNL Report No. 47087, 1992). Short irradiations would obviate the inconvenience and discomfort to the patient of long and often awkward positioning of the head at a reactor port. This advantage alone would justify a clinical use for BNCT, if palliative results on the tumor were at least as favorable as those following the presently, available standard, 6-week, 30-fraction postoperative linear-accelerator-based photon radiation therapy.
Efforts have been made to synthesize porphyrins for the diagnosis, imaging and treatment of cancer. In U.S. Pat. No. 4,959,356 issued to Miura, et al. (which is incorporated herein in its entirety), a particular class of porphyrins was synthesized for utilization in the treatment of brain tumors using BNCT. The porphyrins described in that patent are natural porphyrin derivatives which contain two carborane cages at the 3 and 8 positions. Natural porphyrins have particular substitution patterns which are, in general, pyrrole-substituted and asymmetric. The porphyrins described in U.S. Pat. No. 4,959,356 use heme, the iron porphyrin prosthetic group in hemoglobin, as a chemical starting material; therefore, the resulting boronated porphyrins resemble heme in their basic structure. In contrast, the porphyrins of the current invention are synthetic tetraphenylporphyrin (TPP) derivatives that are symmetrically substituted at the methine positions. Most are also substituted at the pyrrole positions of the macrocycle. Acyclic precursors are used as chemical starting materials so that final product yields are generally greater than those obtained from natural porphyrin derivatives.
U.S. Pat. No. 5,877,165 issued to Miura et al. (which is incorporated herein in its entirety) is focused on boronated tetraphenyl porphyrins containing multiple carborane cages which selectively accumulate in neoplastic tissue and which can be used in cancer therapies such as boron neutron capture and photodynamic therapy.
U.S. Pat. Nos. 5,284,831 and 5,149,801 issued to Kahl, et al. describe another type of porphyrin and their uses in BNCT, PDT and other biomedical applications. Like the porphyrins described in the previous patent by Miura et al., these are also natural porphyrin derivatives but they contain four carborane cages at the 3 and 8 positions.
U.S. Pat. No. 4,500,507 issued to Wong describes a method of labeling hematoporphyrin derivatives (HPD) with 99mTc as a means of visualizing tumors using scintigraphic noninvasive imaging techniques such as SPECT. The method taught by this patent utilizes hematoporphyrin compounds that are also natural porphyrin derivatives.
U.S. Pat. No. 4,348,376 to Goldenberg, U.S. Pat. No. 4,665,897 to Lemelson, and 4,824,659 to Hawthorne teach combining labeling of an antibody with 10B and with one or more other radionuclides, including those of iodine, for purposes of imaging tumors noninvasively and thereby delineating tumor targets for exposure to thermalized neutrons. Each of these patents requires that the 10B compound be linked to a radiolabeled antibody.
Improvement in the efficacy of conventional radiotherapy using chemical agents is a key area of interest in experimental radiation oncology. Currently, more than 750,000 patients in the U.S. receive photon radiation therapy for cancer per year. Success has been limited due to restriction of the tumor dose to avoid critical normal tissue morbidity. Hypoxic cells in tumor can be a major problem because they are three times less sensitive to photon radiation than oxygenated cells. While a whole range of hypoxic cell radiation sensitizing agents have been developed, most have proven clinically ineffective. Accordingly, there is a need for effective hypoxic cell radiation sensitizing agents.